My invention relates to a magnetic tape apparatus of the type having a rotary head assembly for reading, writing, and monitoring data on magnetic tape. More particularly, my invention pertains to the improved arrangement of read/write heads and monitor heads included in the rotary head assembly. Typically, the apparatus according to my invention represents an adaptation of a digital audio tape (DAT) deck for the storage and retrieval of digitized data on the DAT, with a read-after-write monitoring capability, for use as part of a computer system.
Japanese Unexamined Patent Application No. 62-177701, laid open to public inspection on Aug. 4, 1987, represents an example of known rotary-head magnetic read/write system providing for substantially concurrent recording and monitoring of digital data on a DAT cassette. It teaches the use of a rotary head assembly including a rotary drum to which there are mounted not only a pair of read/write heads but also a pair of monitor heads. The pair of read/write heads are disposed in diametrically opposite positions on the rotary drum. The pair of monitor heads are also disposed in diametrically opposite positions on the drum. An angular difference of 90 degrees exists between the positions of the two pairs of heads. The magnetic tape is wrapped around the rotary head assembly through an angle of, typically, 90 degrees.
In this prior art system, as the read/write heads create a series of slanting record tracks on the tape, the monitor heads substantially concurrently scan the tracks for monitoring the recorded data. In event the output signals of the monitoring heads indicate that data is not being recorded properly, the host computer commands the rewriting of the data.
There has been a problem left unsolved in connection with such a rotary-head DAT (R-DAT) system having a substantially concurrent monitoring capability. In R-DAT systems in general, the pair of read/write heads alternately form tracks so as to overlap the preceding track. Such "overwriting" results in the creation of record tracks of reduced width. However, since the monitor heads scan the tracks before they are overwritten, the record tracks eventually formed by overwriting are narrower than those scanned by the monitor heads.
Admittedly, the prior art R-DAT system enables the evaluation of the quality of the recording on the tracks before overwriting. The monitor heads have so far been so arranged on the rotary drum as to scan the complete width of the tracks as formed by the read/write heads. For this same reason, however, the prior art system has often failed to ascertain whether the data has been recorded on the overwritten tracks with a sufficient quality margin or not. This shortcoming can bring about serious consequences because, as is well known, the intensity of the recording on magnetic tape decreases from about five to fifteen percent with the lapse of time through self-demagnetization. Therefore, if the data on magnetic tape is to be monitored substantially concurrently with writing, it is essential to make sure that the quality margin of the recording on the overwritten tracks is sufficient to overcome the self-demagnetization.